Just to throw my hat into the ring here -- Winamp has an excellent media
library, and a history of encouraging interoperability.

Their database stores play-time info, and everything else I've seen
discussed here. All we need to do is mirror the file onto the Archos and
ignore entries that aren't on the MP3 player -- let winamp keep up to date
on the rest. In terms of modifying the file, only the playcount,
last-played and rating entries need to be modifiable on the Archos.

To find this truly excellent tool, just mash the "Media Library" button in
Winamp 5. It lets you do some great dynamic playlists, which are
essentially stored SQL.